Various liquid crystal display modes as well as a TN (twisted nematic) mode and an STN (super twisted nematic) mode have hitherto been proposed. In particular, a GH (guest-host) display system in which a dichroic dye is used in a dissolved state has been spreading as a display panel in automobiles, and the like with its wide angle of vision being taken advantage of.
Dichroic dyes to be used in a GH mode are required to have excellent dichroism, solubility in liquid crystal compounds, light resistance, and coloring power.
Azo dyes having a biphenyl skeleton exhibit a high absorption coefficient (coloring power). They can be prepared at low cost because an azo structure can be introduced on each side of the biphenyl skeleton simultaneously. Further, azo dyes having a long molecular length sufficient for use as a dye for liquid crystals can be obtained with ease. Despite these advantages, these azo dyes have been limited in both production and application due to high toxicity (carcinogenicity and mutagenicity) of the starting benzidine compounds.
U.S. Pat. No. 4,610,803 discloses various dichroic dyes having an azo group on each side of a biphenyl skeleton, but neither does it have mention of specific examples of dyes synthesized from 2,2'-ditrifluoromethylbenzidine used in the present invention nor does it refer to safety in production of such dyes. Moreover, although the above patent covers over a broad range of compounds, the specific examples shown are limited to those dyes synthesized by using phenolic coupling components without revealing the characteristics, e.g., dichroism, of dichroic dyes obtained by using alkylaniline type coupling components, which are typical coupling components other than the phenolic ones.